26 



THE POPULAR SCIENCE MONTHLY 



fications of these intra-cellular movements and to abnormal types of 

 cleavage and development — in short, these movements show that the 

 fertilized egg is differentially sensitive. 



In the further course of development particular portions of the 

 embryo become especially sensitive to some kinds of stimuli, while other 

 portions become sensitive to others. In this way the different sense 

 organs, each especially sensitive to one particular kind of stimulus, arise 

 from the generalized sensitivity of the oosperm, and thus general sensi- 

 tivity, which is a property of all protoplasm, becomes differential sen- 

 sitivity and special senses in the process of embryonic differentiation. 

 Such sensitivity is the basis of all psychic processes : sensations are the 

 elements of the mind. 



3. Tropisms, Reflexes, Instincts. — All the responses of germ cells, 

 and of the simplest organisms, to stimuli are in the nature of tropisms 

 or reflexes, that is, relatively simple, automatic responses. Such tro- 

 pisms or reflexes are seen in the movements of bacteria, protozoa and 



19 



a 



* ■ ■ . i ~ 



\ I 



19* ' 



19' 



■ ■ i i n ... ■ ■— 1 1 - ■■■ i 



= *"*' : -"- w -"*-""- -~- ~- 



2G '■ 



38' 



10- 



25°- 



Fig. 19. Reactions of Paramecium to Heat and Cold. At a the infusoria are 

 uniformly distributed in a trough, both ends of which have a temperature of 19° ; at 

 ft the infusoria are shown collected at the cooler end of the trough ; at c they have 

 collected at the warmer end of the trough. (From Jennings, after Mendelssohn.) 



many higher animals and plants as well as in movements of spermatozoa, 

 the movements of the protoplasm in egg cells and embryonic cells, 

 the movements of cells and cell masses in the formation of the gastrula, 

 alimentary canal, nervous system and other organs. Indeed the entire 

 process of development, whether accompanied by visible movements or 

 not, may be regarded as a series of automatic responses to stimuli. 



